The slew rate of an input signal can be determined by utilizing a comparator to compare the input signal to various thresholds and generating output signals when the thresholds are crossed. The time between the generation of the output signals is the slew rate of the input signal.
FIG. 1A illustrates an example input signal (VIN) and the output signals generated based thereon. The input signal proceeds from ground to VCC over a period of time. When the input signal crosses over a first threshold (e.g., 20% VCC) a first (low) output signal (VOL) is generated and when the input signal crosses over a second threshold (e.g., 80% VCC) a second (high) output signal (VOH) is generated. The time (Δt) between the output signals is the slew rate of the input signal.
FIG. 1B illustrates an example slew rate detector 100. The detector 100 includes two operational amplifier comparators (op-amps) 110, 120. The first op-amp 110 compares the input signal to the high reference voltage (80% VCC) and generates a high out signal when the input signal crosses over the high reference voltage. The second op-amp 120 compares the input signal to the low reference voltage (20% VCC) and generates a low out signal when the input signal crosses over the low reference voltage. The time delay between two outputs (Δt) corresponds to the slew rate of the input signal.
The op-amps 110, 120 need to be matched for the detector 100 to work effectively. However, the op-amp 110 may utilize negative channel transistors (e.g., NMOS) and the op-amp 120 may utilize positive channel transistors (e.g., PMOS). Furthermore, the op-amps 110, 120 may be susceptible to process, voltage and temperature (PVT). Mismatches in the clock to output time (TCO) of the op amps 110, 120 may cause an unacceptable detection error. As the incoming slew rate gets smaller as the speed of applications increase the tighter the detection parameters will be.